The present invention relates to a SQUID-magnetometer for a device for single or multi-channel measurement of very weak magnetic fields caused by at least one field source to be detected, with at least two superconducting gradiometer loops and two Josephson tunnel elements which are connected to the gradiometer loops electrically, forming a d-c SQUID and are arranged in common with the latter on a surface of a rigid carrier. Such a magnetometer is known from the publication "IEEE Transactions on Magnetics", Vol. MAG.-19, No. 3, May 1983, pages 648 to 651.
For measuring very weak magnetic fields, use of superconducting quantum interferometers, also called "SQUIDs" (abbreviation for: "Superconducting QUantum Interference Devices") is generally known ("J. Phys. E.: Sci. Instrum.", Vol. 13, 1980, pages 801 to 813, or "IEEE Trans. Electron Dev.", Vol. ED-27, No. 10, October 1980, pages 1896 to 1908). In the field of medical technology, magnetocardiography or magnetoencephalography is therefore considered a preferred field of application for these interferometers since the magnetic fields caused by magnetic heart or brain waves cause field strengths in the order of only about 50 pT or 0.1 pT. ("Biomagnetism- Proceedings Third International Workshop on Biomagnetism, Berlin 1980", Berlin/New York 1981, pages 3 to 31). It is necessary, however, to detect these fields in the presence of relatively large interference fields.
For measuring biomagnetic fields in the mentioned order of magnitude, measuring devices are known which can be constructed single-channel and in particular, also multichannel (see, for instance, DE-OS 32 47 543). Depending on the number of channels, these devices contain at least one SQUID magnetometer with a first or higher-order gradiometer.
Such magnetometers are shown in the literature reference "IEEE Trans. Magn." mentioned at the outset. In a special embodiment with a first-order gradiometer, a double loop of superconducting conductors in the approximate shape of an "8" is provided. In a common connecting line of the two loops of this double loop, two Josephson tunnel elements are integrated which results in the characteristic design of a d-c SQUID. For forming second or higher-order integrated d-c SQUID magnetometers, the two loops of the double loop of the known first-order gradiometer can each be replaced by a corresponding number of double loops. All superconducting parts of these known SQUID magnetometers are placed here on one flat side of a plane carrier. While good suppression of interference fields is possible with such a planar SQUID magnetometer, it is relatively insensitive since with the two immediately adjacent loops of its double loop only the gradient of the magnetic flux to be measured is detected, but not the flux itself. It is, however, an advantage of a planar SQUID magnetometer that it is relatively easy to manufacture.
Besides such planar SQUID magnetometers, there are also known gradiometers which have a pronounced three-dimensional shape (see, for instance, the publication "Rev. Sci. Instrum.", Vol. 53, December 1982, No. 12, pages 1815 to 1845 or European Patent Application No. 0, 184, 670). The gradiometers form here a so-called flux transformer which contains at least two gradiometer loops. The loop facing the magnetic field source to be detected is also called here the detection loop, while the loop farther removed therefrom can be considered a compensation loop. With this loop arrangement, the magnetic flux of the field source can advantageously be measured directly, where very high sensitivity and good discrimination of external interference fields can be achieved. In such flux transformers, however, the detected flux is always coupled into a SQUID loop inductively via a coupling loop connected to the gradiometer loops. Because of the losses connected therewith, the gradiometer loops must have relatively large dimensions. This, however, leads to an undesirably large inductance of the individual loops.